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Current time:0:00Total duration:13:25

Video transcript

so we've talked about pacemaker cells and I thought it'd be kind of neat to to draw out again exactly what these pacemaker cells do and compare them to one another so we know we have different types of pacemaker cells and I'm going to use our millivolt scale just as we usually do to kind of compare them so the first type is our SA node so let me put that up here SA node and those cells start out negative and then they kind of slowly creep up positive and try to remember why that happens you know that the main reason for that is that you have the increasing sodium permeability so at this point more sodium is kind of rushing into the cell sodium is getting in and if it's getting in quickly it's let's say it's like just gushing in then that line would be very very steep you know something like this but if it was kind of getting in very slowly it would be a little bit more shallow so I've drawn it kind of the way that it usually is drawn kind of somewhere in between and when that sodium permeability hits or when that cell hits a certain threshold let's say somewhere in there then it's going to fire an action potential right so this is our thresholds for firing and and that just means that the channels for calcium open up and so they open up and make the cell go positive and then eventually the potassium channels take over and it becomes negative again and this part right here then we think of as the action potential so this is what it looks like for the SA node but let me now do it again for the AV node so if there's an AV node it's going to kind of get up to that threshold a little bit more slowly but once it gets there it's going to look basically the same as the SA node really no different and it's going to come down again so this might be the AV node and finally we have let's say a cell that takes even longer to get to threshold this would be like the bundle of His and again it looks basically the same once it does get there though so these action potentials don't really look that different from one another but the amount of time it takes to get to threshold changes because the the bundle of his cells for example they are going to be least permeable to sodium and a Navy know to be somewhere in between and the SI notre most permeable the sodium so that's why those lines are slightly less steep as you go along so this is how it looks and the key difference here is that you're basically extending this heartbeat out right this is one heartbeat and this would be like the SA node heartbeat but if your AV node was controlling your heartbeat it might take a little bit longer something like that and if your bundle of hiss was taking control of your heart the heartbeat would be really long something like that so it would take longer longer for the heartbeat in terms of time depending on which part of the electrical conduction system is in charge so let's actually think about that a little bit more carefully so let's say we have our SA node and you're talking about heart beats let's actually write out let's say heart rate heart rate is H R and this is going to be in beats per minute per minute and then let's say I flipped it around and wanted to know how long one beat takes so one beat and that will probably have to be done in seconds so how would that be for the SA node well we know that the SA node and this is just a number out of books you can find them saying something like 60 to 90 beats per minute and if we took the upper range let's say it took 90 and try to figure out how long one beat would take you would say okay you have one minute gives you 90 beats I'll put B for beats and then you have one let's say minute is 60 seconds right and so the minutes cancel and now you're left with zeros cancel two thirds of a second all right so two thirds of a second per beat and actually I might even like to I'm going to erase two thirds and just rewrite that as 0.66 okay this point is six-six something like that so that's how long it takes for the SA node to to fire off one beat and in fact just a really hammer home the point that's this distance right here right that's 0.66 seconds so now for the AV node we could do the exact same thing we can say well the AV node we know usually somewhere between forty to sixty beats and I'm going to use that number and this one's really easy right because if it's 60 beats a minute that means that one beat is one second that was actually a really quick one so that's one second and finally for the for the bundle of hiss the bundle of hiss I'm going to write that as B Oh H again bundle of hiss is going to be somewhere between let's say twenty to thirty beats per minute and if we use that the number thirty that means that you get a beat every two seconds so every two seconds this will go off and I know that my picture now if since you know those numbers it's not going to look as impressive because I should have drawn the bundle of His even more stretched out than it is but just assume that that's two seconds on that graph so if that's the case now let's let's kind of jump back to how we usually think about our heart and the fact that you've got four chambers right and the conduction system is actually going to go through all of that and starts here in the SA node goes down to the AV node and then you've got the bundle of hiss somewhere down there and you've got connections down there and you might be thinking well wait a second you haven't drawn in all of the rest of the electrical electrical conduction system and that's true but for right now let's just focus on these three parts right so you got a V here and you've got the bundle of hiss over here boah there so you've got these three parts and they're kind of spaced out right like this is two centimeters apart let's say I'm just kind of guesstimating and this might be let's say even closer let's say one centimeter so these are kind of anatomically how they're laid out in terms of how far apart they are from each other so the question might come up you know how exactly do you do explain the fact that it's always the SA node that fires off right it's never you av node or bundle of hits we always say well he's in sinus rhythm right and what does that mean well son says someone is in sinus rhythm all they're saying is that the SA node is what's controlling their rhythm so sinus rhythm you might hear that actually a lot on TV shows I've noticed they like to throw that term around and it just means that you're in a rhythm controlled by your SA node so how does that work exactly because if it's firing every point six six seconds that's fine but how come these two other you know pacemaker cells aren't ever firing well it gets back to basically trying to beat them out so if you can get a signal from your SA node this is your let's say your SA node kind of from that that drawing above this is your SA node if you can get it to your AV node faster if you can get that signal there faster then it the Nate would fire then you've beat it out so basically if you can get there a signal from the SA node over to the AV node if this happens in less than what is that one second then the AV node is not going to get a chance to fire before you're already firing for it so this is the race right the SA node is basically trying to get a signal over there quickly and these distances that it has to cover we set about two centimeters in about one centimeter so so what is the math I how does that work out so you can actually look up these numbers and it turns out that if you check it out these conduction velocities are really really fast right so it's about 0.5 meters per second up here and it gets even faster as you get along further so it's about 2 meters per second here so these are the velocities of the signal how fast the electrical conduction system is actually sending along that signal and those are the distances so if you if you think that if you just multiply them you should be able to get a time how long it will take a signal to get from the SA to the AV node so we know that the SA node fires every 0.66 seconds right that much we figured out already so the question is can it get a signal to the AV node before the AV node fires by itself can it get a signal down there in less than one second you're trying to beat out this time and can it get a signal to the bundle of hiss in less than two seconds you're trying to beat out that time as well so let's let's figure it out so this math works out too let's see we got 0.5 meters and you're going to want to end with a time so I'm going to put one second up here and you have let's say one meter is a hundred centimeters so the meters cancel and you've got two centimeters to cover so the centimeters cancel so you've got two two divided by what is that 50 right and that's seconds so it's zero point six six seconds plus 125th that's 125th let me make a little bit more space on here just it just so it doesn't feel as crowded there we go so 125th of a second is the same as 0.04 seconds and that is 0.7 seconds so so far the signal has gotten here in 0.7 seconds I'm just going to write that in yellow because this is the SA known signal zero point seven seconds wow that's really fast right really fast let's see how long it takes to get to the bundle of His so to get to the bundle of his so I'll do that math over here you have to now add up 0.7 seconds because that's how long it took to get to the AV node and then you have to add 0.1 seconds and what is that for this my friends is the delay this is the delay of the AV node remember the AV node creates this delay so that the ventricles contract just a little bit after the atrium is due so this delay is actually built into the system the delay is about a tenth of a second and then you have to figure out how long it takes to travel so how long does it take to travel that last little bit well it's going to be going - I'm sorry one second it covers two meters and we know that one meter has 100 centimeters and we know that we're trying to cover one centimeter so the centimeters cancel meters cancel and you're left with 1 over 200 seconds so that's how long it actually takes to travel travel time you can think of it as so that would be 0.005 seconds so in total it's now taking us zero point eight zero five seconds so this is how long it takes to get to the bundle of hits so now let me write that up here zero point eight zero five seconds now we're really happy because we were able to beat out both the AV node and I guess from the perspective of the SA node if SA node cells got happy that's what they would look like so it basically gets it really really quickly is the point so point seven seconds in point eight oh five seconds so that explains at least why you never really see the AV node or the bundle of his cells firing right now going back up here imagine for a second that you actually had a problem let's say you actually had some sort of disease or some sort of issue with your cells and let's say these SA node cells gave up right well if they gave up then no signal would be coming into the AV node and the AV node becomes your plan B this is your plan B the SA node of course that's your plan a right that's what you're usually doing but it's nice because you have this plan B and if a second one second goes by then it without a signal in the AV node kicks in and that'll start firing and you'll have a new heart rate something closer to forty to sixty but at least your heart is beating now let's say you know catastrophe strikes and you're a V node is down to well your bundle of hiss is your plan C your plan C and so now if two seconds go by in a bundle of hiss have not gotten a signal then they start firing and your heart rate will be somewhere between twenty and thirty so these are the kind of backup mechanisms your heart has to make sure it always beats and this is one of the neat things that your heart is figured out to create not just a plan a but also a plan B and a Plan C